I've been reviewing PhpStorm (awesome, by the way) for our company and was tearing my hair out trying to get xdebug working on Ubuntu - it kept saying over and over that port 9000 was busy. Maybe some other bright mind would have jumped to this conclusion right away but I finally did a netstat -pant and saw that Zend Studio was already listening on port 9000. All well and good, except that I was not currently debugging with Zend Studio!
So this is a bit of knowledge contribution: if you have Zend Studio even RUNNING (and I'm sure its the same with Eclipse PDT) , it will completely take over port 9000. Only run one IDE at a time.